Wastewater treatment is one of the most important and challenging environmental problems associated with coal-based power generation. Using wet scrubbers to clean flue gas is becoming more popular worldwide in the electrical power industry. In the coming years, hundreds of wet scrubbers will be installed in the U.S. alone. While wet scrubbers can greatly reduce air pollution, toxic metals in the resulting wastewater present a major environmental problem, and the energy industry will be investing billions of dollars to meet increasingly stringent environmental regulations. Cost-effective and reliable technologies capable of treating such complicated wastewater are in demand.
Zero-valent iron systems are known to be effective for reducing the concentration of contaminants in wastewater streams. Among zero-valent iron systems are hybrid zero-valent iron (hZVI) systems in which the iron corrosion process is utilized to transform and immobilize various heavy metals and reactive anionic contaminants in wastewater. In the hZVI system, an activated iron media is created and maintained to treat contaminated waters. The activated iron media includes three components: zero-valent iron (Fe(0) or ZVI) particles having at least a partial magnetite (Fe3O4) coating, discrete magnetite particles, and ferrous ion (Fe2+) in solution in the environment of the particle components. Some ferrous ion may be adsorbed onto the solid surface of the particles and become surface-bound Fe(II). The ferrous ion in solution plays a central role in preventing the formation of ferric oxides during the iron corrosion process, which occurs due to the presence of oxidizing compounds in the water, such as dissolved oxygen, nitrate, and selenate, among others. In the hZVI process, discrete magnetite particles acquire electrons from ZVI particles and become electron-enriched reactive magnetite that can react with various contaminants by delivering electrons to the target contaminants (i.e., magnetite affects contaminant reduction). Thus, the discrete magnetite particles host the redox reactions and play the role of electron shuttle.
In the hZVI system, ZVI is the primary electron source. With its magnetite coating, the ZVI particles also serve as reaction sites for various redox reactions.
The various roles of ZVI in the hZVI system suggest that ZVI particle size may affect the system performance. For example, under the same ZVI concentration (e.g., 100 g/L), the use of a smaller ZVI particle size means that a higher surface area is available for hosting the reactions for contaminant transformation and immobilization. Moreover, the higher specific surface area also means that more effective electron transfer between ZVI and discrete magnetite particles, thus high surface area ZVI may be expected to be more efficient in generating reactive magnetite particles and thereby indirectly support contaminant removal. Overall, it may be postulated that smaller ZVI particle size may increase hZVI system performance.
Alternatively, other factors may need to be considered: Examples of these other factors include (1) price, fine ZVI source particles are more expensive than coarse ZVI source particles, (2) safety, extra-fine ZVI source particles may be too reactive to be handled safely, and sub-micron size ZVI source particles may pose a risk of explosion or as a self-inflammable hazard, and (3) fine particles may be more difficult to settle and thus may not be compatible with a typical hZVI reactor design for rapid solid/liquid separation.
Despite the advances in hZVI technologies to date, a need exists for improved hZVI systems and methods for more effectively reducing the concentrations of contaminants in wastewaters. The present invention seeks to fulfill this need and provides further related advantages.